1. Field of the Invention
The present invention generally relates to a starting control method for a brushless direct current (BLDC) motor and, more particularly, to a sensorless starting control method for a BLDC motor.
2. Description of the Related Art
In recent years, electric motors have been taking an important role in a variety of industrial applications. For example, a cooling fan is usually equipped in an electronic device for heat dissipation. Based on this, the BLDC motor has been widely used in the cooling fan to control the impeller rotation of the cooling fan so as to improve the cooling efficiency of the electronic products.
In some occasions while operating the BLDC motor, a Hall sensor is generally used to detect the locations of magnetic poles of a rotor in advance so as to control the rotation of the rotor. In some applications, however, the Hall sensor may become useless due to the operation environment. For example, in an environment where a compressor is operated with an extreme high temperature, the Hall sensor tends to malfunction easily due to the high operation temperature generated by the compressor.
There have been sensorless starting control methods proposed for solving the problem described above, as described below. Please refer to FIG. 1, a sensorless starting control method for a traditional BLDC motor is shown. The method comprises a rotor-positioning step S91, an open-looped starting step S92 and a close-looped operation step S93.
Please refer to FIGS. 2 and 3a, a three-phased BLDC motor 9 is used as an example for illustration purpose. The BLDC motor 9 has six stator magnetic poles 91 and a rotor 92 having four rotor magnetic poles 921. Each of the stator magnetic poles 91 is wound with a respective one of three-phased coils u1, v1, w1, u2, v2 and w2.
Please refer to FIG. 2 again, a three-phased full-bridge inverter comprising six electronic switches SW1 to SW6 is disclosed. During the excitation of the coils u1, v1, w1, u2, v2 and w2, the BLDC motor 9 may control the direction and the amplitude of a current passing through any one of the coils u1, v1, w1, u2, v2 and w2 via the three-phased full-bridge inverter.
Please refer to FIGS. 2, 3a and 3b, during the rotor-positioning step S91, one of the coils u1, v1, w1, u2, v2 and w2 is excited by a supply voltage in order for the rotor 92 to be positioned in a stator starting position P1. More specifically, by turning on the electronic switches SW1 and SW2 for a time interval X1 (with references to FIGS. 2 and 3b), the coils u1 and u2 are excited by the supply voltage so that an N-pole magnetic field is generated (with reference to FIG. 3a). Similarly, the coils v1 and v2 are also excited by the supply voltage so that an S-pole magnetic field is generated. As a result, adjacent two of the rotor magnetic poles 921 with different magnetic poles may be magnetically attracted by the N-pole magnetic field generated by the coil u1 and the S-pole magnetic field generated by the coil v1, driving the rotor 92 to rotate by a small angle until a rotor magnetic pole border D1 where the adjacent two of the rotor magnetic poles 921 border each other is aligned with the stator starting position P1. The stator starting position P1 is located between two stator magnetic poles 91 respectively wound with coils u1 and v1, as shown in FIG. 3a. 
Please refer to FIGS. 3c to 3f, during the open-looped starting step S92, each of the coils u1, v1, w1, u2, v2 and w2 is excited by the supply voltage in sequence based on a plurality of driving time intervals in order to drive the rotor 92 to rotate in a predetermined direction. Specifically, the three-phased full-bridge inverter having the electronic switches SW1 to SW6 in FIG. 2 is driven, with the electronic switches SW2 to SW6 switched in turn based on the driving time intervals Y1 to Y4 so as to control the directions of the currents passing through the coils u1, v1, w1, u2, v2 and w2. In the step, each coil of the BLDC motor 9 is excited in sequence, enabling the rotor 92 to rotate in the predetermined direction. As such, referring to FIGS. 3c to 3f, the rotor magnetic pole border D1 of the rotor 92 rotates in a counterclockwise direction through a first starting position Q1, a second starting position Q2, a third starting position Q3 and a fourth starting position Q4, thus creating a back electromotive force (EMF).
In the close-looped operation step S93, a controller 93 controls a close-looped rotational speed of the BLDC motor 9 based on a feedback of the back EMF. More specifically, referring to FIG. 2, the back EMF is sent to the controller 93 via a detection circuit 94 so as to control the BLDC motor 9 to rotate in a constant speed after the rotational speed of the BLDC motor 9 has achieved a predetermined level. According to the steps S91 to S93, the sensorless starting control method for the BLDC motor 9 is provided.
In general, the above sensorless starting control method has some drawbacks as described below. Referring to FIG. 4, during the rotor-positioning step S91, the magnetic field generated by the coils u1, u2, v1 and v2 has the same polarity as that generated by the rotor magnetic poles 921, causing a rotation dead angle of the motor. In this case, the magnetic force generated by the coils u1, u2, v1 and v2 not only has the same magnitude as the magnetic force generated by the rotor magnetic poles 921, but also with opposite direction to the magnetic force generated by the rotor magnetic poles 921, causing the two magnetic forces to be offset by each other. As a result, the rotor magnetic pole border D1 of the rotor 92 can not be aligned with the stator starting position P1, leading to a failure of the subsequent open-looped starting step. In other words, the rotor 92 is not able to rotate through the starting positions Q1, Q2, Q3 and Q4 during the driving time intervals Y1 to Y4, causing an abnormal back EMF to be generated. As a result, the controller 93 operating based on a feedback of the abnormal back EMF fails to start the BLDC motor 9.
To solve the problem, a conventional method is to increase the supply voltage of the BLDC motor 9 in order to increase the starting torque of the BLDC motor 9. Although this method efficiently overcomes the problem of rotation dead angle of the BLDC motor 9, it significantly increases the power consumption.
Besides, once the rotor 92 is positioned in a dead angle where the angle difference between the rotor magnetic pole border D1 and the stator starting position P1 is 90 degree as shown in FIG. 4, the BLDC motor 9 is likely to rotate in a direction opposite to the predetermined direction during the open-looped starting step S92 when the supply voltage of the BLDC motor 9 is increased for performing the rotor-positioning step S91. Thereafter, as the conventional method proceeds to the close-looped operation step S93 as controlled by the controller 93, a back EMF generated by the BLDC motor 9 rotating in the direction opposite to the predetermined direction could sometimes be the same as a feedback control value preset in the controller 93, and the BLDC motor 9 may therefore keep on rotating in the wrong direction because the rotor 92 has been incorrectly determined to be operated in a normal condition by the controller 93. With the improper operation of the BLDC motor 9, the user may need to manually reset the BLDC motor 9, making it more inconvenient for operating the BLDC motor 9. Therefore, there exists a need to improve the sensorless starting control method for the BLDC motor 9.